The present invention relates to gas turbine operation and, more particularly, to a system and method for controlling gas turbine operation in a closed-loop manner based on estimated stress levels at key locations within the turbine.
Typical gas turbine operating control schedules are open-loop in nature, derived from extensive analysis of simulated thermal and mechanical stress levels in rotating components, and designed so that these nominal stresses are not exceeded during operation. As an example, a possible startup schedule for a gas turbine is shown in FIG. 1. The schedule includes turbine speed versus time and is used by the turbine speed controller as a reference. As the machine accelerates from startup, both mechanical and thermal stresses build up. Mechanical stresses are primarily due to aerodynamic reactions as well as rotational and centrifugal forces. Thermal stresses arise from differential thermal expansion within turbine metal parts. These thermal stresses result from sources of heat within the turbine that are not uniform, and hence different metal parts heat up at different rates. When two parts that are secured together expand at different rates, or even a single part that is massive enough that separate regions of the part expand at varying rates, mechanical deformation and severe stressing may result. Once these parts attain a substantially uniform temperature, however, the stress levels decrease.
Since peak stress levels cannot be allowed to exceed limits dictated by material integrity as well as ultimate component life, it is important that the machine is operated in such a manner that the stress levels are kept below these limits at all times. In the case of machine startup, this is achieved by xe2x80x9choldingxe2x80x9d the turbine at certain predetermined points in its startup cycle to allow the heat to xe2x80x9csoakxe2x80x9d in. FIG. 1 shows two such hold points at 50% and 85% of full speed. Hold points and hold times are typically derived from extensive off-line analysis that attempt to predict stress patterns using accurate, but very high order finite-element models.
To account for machine-to-machine variations as well as inaccuracies in the models, safety margins are built into the operating schedules. Better performance could be obtained from the machine in terms of quicker startups and the like if stresses could be measured or estimated on-line. Measuring such stress levels on rotating components, however, is prohibitively expensive.
In an exemplary embodiment of the present invention, a method of operating a gas turbine includes the steps of (a) measuring at least one measurable temperature (TMEAS) in the gas turbine; (b) using heat conduction and convention equations to estimate a first critical temperature (T1) and a second critical temperature (T2) based on TMEAS; and (c) controlling the gas turbine based on T1 and T2.
In another exemplary embodiment of the invention, a method of estimating critical stress in a gas turbine includes the steps of (a) measuring at least one measurable temperature (TMEAS) in the gas turbine; (b) using heat conduction and convection equations to estimate a first critical temperature (T1) and a second critical temperature (T2) based on TMEAS; and (c) estimating the critical stress in real time according to a stress model prediction based on the difference between T1 and T2.
In still another exemplary embodiment of the invention, a system is provided for estimating critical stress in a gas turbine. The system includes a probe that measures at least one measurable temperature (TMEAS) in the gas turbine. A processor receives input from the probe and uses heat conduction and convection equations to estimate first and second critical temperatures based on TMEAS. The processor includes a memory storing a stress model prediction algorithm and estimates the critical stress in real time based on a difference between T1 and T2 using the stress model prediction algorithm.